Many communications networks today provide high bit-rate transport over a shared medium, such as a Passive Optical Network (“PON”), a Cable television (“CATV”) coaxial or hybrid fiber/coax network, or a wireless network. These shared medium networks typically use time, frequency or code division multiplexing to transport data signals from a central terminal to several remote customer terminals and Time Division Multiple Access (“TDMA”) to transport data signals from the remote terminals to the central terminal. TDMA is characterized by non-continuous or burst mode data transmission.
In existing optical networks, especially in a PON system, each “packet” of information from a remote terminal is multiplexed in a time sequence on one fiber and transmitted in a burst-like manner. Such burst mode packets transmitted from one or more remote terminals to a receiver at the central terminal present a number of constraints to the receiver. The receiver is likely to receive closely spaced packets that have a wide range of optical power levels. A weak signal in a received packet needs to be amplified sufficiently so that it can be used, but if a strong signal in a packet is amplified too much the signal can be lost due to blinding (saturating) the amplifier. The difference in power levels of the incoming packets may be such that the noise induced on a powerful signal in one packet may be greater than a weak signal in a subsequent packet.
In burst mode, a guard time is provided between packets. Typically, the guard time is the time to transmit 8 bits. For example, at 6 nano-seconds (“ns”) per bit, the time to transmit 8 bits is 48 ns. Thus, the receiver must be able to respond to a packet with a different power level in less than 8 bit times. The power ratio between sequential packets can be as much as 1000:1. The receiver also has to respond to a weak signal in a subsequent packet within the guard time between packets.
An incoming packet can include a long string of bits set to logic ‘one’ or logic ‘zero’. The receiver has to maintain the output at a constant logic level while the string of bits set to logic ‘one’ or logic ‘zero’ is being received. However, the string of bits set to logic ‘one’ or logic ‘zero’ may be longer, up to three times longer or more, than the guard time between packets.
A burst mode receiver can be AC coupled or DC coupled. Prior art AC coupled receivers cannot maintain the voltage level for a long string of ‘ones’ or ‘zeros’ in a packet and also respond to a weak signal in a subsequent packet within the guard time between packets. Therefore, the AC coupled receiver may drop bits in the next packet while adapting to the new power level.
In a DC coupled receiver, a rapidly responding Automatic Gain Control (“AGC”) is used to keep an amplifier from saturating on the noise or logic ‘zero’ power level on a strong incoming signal. However, AGC can produce severe Pulse Width Distortion (“PWD”) after receiving a long string of ‘ones’ or ‘zeros’ in a packet at a high power level. Furthermore, the AGC must be extremely fast. This limits the incoming data rate at which the DC coupled receiver can operate because as the incoming data rate increases, so does the speed required for the AGC.